Double-wall, vented heat exchanger

ABSTRACT

A plate heat exchanger includes a plurality of plate pairs for providing a flow path for two fluids. The plate heat exchanger has an inlet and an outlet for each of the two fluids, wherein facing surfaces of two adjacent plate pairs of the plurality of plate pairs defines a flow path for a first fluid. The opposite surface of one of the two adjacent plate pairs and a facing surface of another adjacent plate pair from the plurality of plate pairs provides a flow path for a second fluid. The first fluid and the second fluid flowing along their respective flow paths are maintained in thermal communication with each other. A predetermined vent path is formed in at least one of the facing surfaces of each plate pair capable of venting each fluid exterior of the heat exchanger.

FIELD OF THE INVENTION

The present invention is directed to a plate heat exchanger, and morespecifically to a plate heat exchanger having double-wall, ventedconstruction for venting a fluid resulting from leakage occurring withinthe plate heat exchanger along a predetermined leak path to apredetermined region along the exterior of the plate heat exchanger.

BACKGROUND OF THE INVENTION

Heat exchangers are traditionally used to heat or cool potable orprocess critical fluids using non-potable fluids while providing aphysical, mechanical boundary to prevent contact between the respectivefluid streams. Heat exchangers, as with all mechanical devices, havetheir respective, unique finite operating timeframes at the end of whichthe devices fail for one or more reasons. A typical failure mode forheat exchangers is a boundary breach that either allows one or bothfluids: 1) to escape to the outside environment or atmosphere (externalleak) or 2) to mix with one another without escaping to the outsideenvironment (internal leak). With heat exchangers used in potable,sanitary, or critical fluid applications, an internal leak that allowsthe two fluids to mix can have catastrophic results, such as illness orpoisoning in the case of potable and sanitary applications, or chemicalfire or explosion in the case of critical fluid process applications.Internal leaks are generally not noticed immediately, whereas externalleaks are usually visually evident.

To avoid this possible situation of having an unseen (internal) leak, itis desirable to provide a fully vented, double-wall boundary thatexhausts the leaking fluid to the outside environment or atmosphere inlieu of having the respective fluids mix inside the heat exchanger whilethe heat exchanger continues to operate. The manufacturing processesrequired to manufacture heat exchangers having double-wall constructionare more intensive, thus double-wall heat exchangers are generally moreexpensive than conventional, single-wall heat exchangers. As a result,in unregulated competitive markets, many single-wall heat exchangers areused in applications that truly require fully vented, double-wallconstruction to provide an adequate level of safety. In an effort toincrease the general public safety, various agencies and governmentalbodies have implemented construction codes that require the use of a“double-wall” heat exchanger or “double-wall, vented” heat exchanger forpotable fluid applications.

For example, UNDERWRITERS LABORATORIES INC.® or UL®, both registeredtrademarks of Underwriters Laboratories, Inc. of Chicago, Ill., hasestablished certification requirements to include double-wall ventedconstruction for all refrigerant to potable water heat exchangers.Examples include water coolers/fountains and refrigerant desuperheaters.Also, IAPMO®, a registered trademark of the International Association ofPlumbing and Mechanical Officials of Ontario, Canada has establishedplumbing code certification requirements to include double-wall heatexchangers for potable water heating, or at minimum, double separationor two heat exchangers. Examples of double separation heat exchangersinclude domestic hydronic or steam boilers used to heat domestic hotwater. In addition, various U.S. states and municipalities and othercountries require double-wall or double separation of the respectivefluids when used in potable water applications.

A double-wall heat exchanger is one in which the heat transfer surfaceseparating the two fluids is comprised of two separate surface layers,rather than one. Thus, if the first surface layer fails to provide afluid tight barrier, the second layer should remain intact, causing anamount of fluid that is in contact with the failed surface layer to flowbetween the surface layers, preferably to a location where the leakingfluid can be detected externally of the heat exchanger so that the heatexchanger can be removed from service. The double-wall construction isintended to be a safety feature to prevent cross-contamination of thefluids.

A further definition of this leak detection process is a “leak path”between the two surface layers. This leak path is either due to aninherent spacing between the metal-to-metal surfaces of the two layers,or an intentionally formed spacing between the two layers, or sufficientporosity between the two layers. In any event, the fluid from the failedsurface layer will follow the “leak path” to an external location fordetection of the leaking fluid.

The first commercially available double-wall heat exchangers toincorporate these operational features were tubular designs. Forexample, U.S. Pat. No. 4,210,199 issued to Doucette et al. is directedto double-wall tubes utilizing two tubes, one tube disposed inside theother tube. The diameter of the outer tube is intermittently reduced, orswaged upon the inner tube, thereby creating the double-wallconstruction, the contact between the walls being provided for heattransfer and a vent path between the fluids. Other similar tubulardesigns have also been used. These methods yield a product that has arelatively accurately controlled vent-path and contact area dimensions.

When the above-mentioned double-wall tube is inserted into a heatexchanger, a “double tube sheet” method is used, in which an outer tubeand an inner tube is each inserted through adjacent tube sheets. A tubesheet is a typical component of tubular heat exchangers through whichthe tubes are inserted and subsequently sealed by means of mechanicalrolling, hydraulic expansion, or welding/brazing. This method provides adouble seal between the inner and outer tubes and a leak path for thetube joints, resulting in a fully-vented double-wall heat exchanger.These tubular designs suffer from at least two main drawbacks: 1) highfabrication cost, and 2) large physical size relative to conventional,single-wall designs.

Double-wall plate-type heat exchangers are a later development. Thedouble-wall plate-type heat exchanger is fabricated by simultaneouslyforming two thin-wall strips of material in the same tool, such thatboth strips are formed together, substantially identically, resulting ina pair of heat exchanger plates that serve as a double layer. Thedouble-layer plate pairs or sets are then stacked to form fluid flowpassages between the plate pairs and are separated from each other platepair via elastomer gaskets disposed along the periphery of the platepairs. The entire assembly is then compressed and held together vialong, threaded bolts that are positioned along the heat exchangerperiphery.

A primary limitation of this construction is that the contact areabetween the two formed plates and the flow area in the vent path (i.e.,leak path) are difficult to control because the tensile properties ofthe plates of the respective plate pairs cause a “spring back” effectthat occurs after the forming process. This spring back effect preventsthe strips from completely nesting together, or forming a substantiallyconformal contact therebetween. The vent path flow area can be extremelysmall when the vent path is adjacent to locations that have contactbetween the substantially conformal adjacent plates, such that elevatedlevels of fluid pressure can be required to force a flow of a leakingfluid through the vent path flow area. These locations of conformalsurface contact between the adjacent plates typically have a relativelyhigh heat transfer coefficient. Other locations between the adjacentplates that are not in conformal surface contact typically have a lowerheat transfer coefficient because of the additional thermal resistancecaused by the spacing between the plate surfaces. The leak path flowarea adjacent to these locations are typically relatively large andthereby offer a relatively lower flow resistance for the leaking fluid.If the designed vent path gap is increased to minimize the amount offluid pressure necessary to force fluid flow between the plates, thenthe coefficient of heat transfer for the double-wall plate issignificantly decreased. This decrease in heat transfer coefficient isdue to the increased thermal resistance associated with the increasedspacing between the plates, the relationship between decreased heattransfer coefficient and increased spacing between the plates being asubstantially linear relationship. The reduction of fluid leakagepressure versus the reduced heat transfer coefficient resulting fromincreased plate spacing is a primary dilemma of plate heat exchangers indouble-wall applications.

An additionally important consideration in the design of double-wallplate heat exchangers involves the regions surrounding the port areaswhere the two fluids are separated by a port seal. To fully meet theintent of the aforementioned building codes requiring double-wallconstruction, the port areas must be fully-vented to the outsideenvironment via a double-port-seal system. In gasketed-type plate heatexchangers, various gasketing methods are used to create double portseal structures that allow a leak to be revealed externally to the heatexchanger if the first port seal fails. U.S. Pat. No. 4,976,313 issuedto Dahlgren provides an example of this technology. However,gasketed-type plate heat exchanger designs suffer from three primarydrawbacks: 1) high fabrication cost, 2) gasket life that is shorter thanthat of all-metal construction types, and 3) lower pressure-bearingcapabilities than the tubular-type exchangers.

Brazed-plate heat exchangers are the latest entry into the double-wallheat exchanger market. Brazed-plate type heat exchangers are similar inconstruction to the gasketed-type plate heat exchangers in that they areconstructed using plates fabricated by simultaneously forming twothin-wall strips of material in the same tool, such that both are formedtogether, substantially identically, as a double layer. Thesedouble-plate pairs or sets are then stacked to form the fluid passages.However, instead of utilizing gaskets and long bolted fasteners toprovide the sealing mechanism, brazed-plate heat exchangers utilize thinsheets of braze material such that the plate pairs braze together in abrazing furnace or other heating device. U.S. Pat. No. 5,291,945 issuedto Blomgren et al. is directed to a double-wall brazed plate heatexchanger.

A critically important manufacturing concern in the manufacture ofdouble-wall, brazed-plate heat exchangers is in preventing the brazematerial from flowing into the vent path between adjacent plates viacapillary action and thereby blocking the flow path for the leakingfluid to escape the heat exchanger. The aforementioned U.S. Pat. No.5,291,945 addresses this problem at the periphery of the plate pairsonly. The solution offered by the heat exchanger construction of thispatent is to provide a sufficiently large spacing between the peripheraledges of the double plate pairs such that both capillary and gravityforces prevent the braze metal from wicking to the small interspace gapsbetween the respective double plate sets, with the heat exchanger platesbeing brazed in a particular orientation to take advantage of thegravity forces.

Another drawback of the heat exchanger construction of U.S. Pat. No.5,291,945 is its lack of pressure-bearing capability. This lack ofpressure-bearing capability is a result of the peripheries of therespective double plate pairs not being brazed, such that the only useof braze material to hold the plate pairs together is limited to theport areas. To increase the pressure-bearing capability of the unit suchthat it can withstand the requirements of relatively low-pressuresystems, a mechanical reinforcement system consisting of a threaded rod,two washers, and two nuts is added to each port structure.

In summary, U.S. Pat. No. 5,291,945 design has several drawbacksincluding:

-   -   i) port holes not having vented, double-seals needed to meet        building code requirements;    -   ii) port areas requiring structural re-enforcement;    -   iii) braze material foil not being stamped at the same time as        the plates;    -   iv) lacking provisions to prevent or encourage braze material        wicking into the vent path;    -   v) lacking a predetermined location for the external leak to        occur; and    -   vi) low design working pressure.

Others, such as SWEP International AB, of Sweden and WTT (WilchwitzThermo-Technik) of Germany, also manufacture double-wall, ventedbrazed-plate heat exchangers. They each separate the double-plate pairsafter the forming process and apply to a portion of the periphery of theplate surfaces a coating, such as an oxide, that repels the capillaryflow of the braze material, and then rebuild the double-plate pairsprior to assembling the heat exchanger for brazing. This step preventsthe entire periphery of the plates from being filled with brazematerial, which would completely close the leak path from venting to theenvironment, and provides an amount of structural reinforcement to theheat exchanger.

However, all of the known double-wall brazed-plate heat exchangerconstructions have a common drawback in that no construction hasprovided a double seal around the ports or a separate vent path aroundthe ports. An inherent assumption behind all known constructions ofbrazed-plate heat exchangers is that if a port joint fails (e.g., brazematerial etches away) the vent area opens up and allows the leak to bevented between the plates. This assumed venting of a leak in the portarea cannot be guaranteed, and thus presents a major obstacle in meetingthe intent of the building and plumbing codes and requirements thatrequire double-wall separation.

In summary, brazed-plate heat exchangers have the inadequacies of: ahigh vent path pressure drop, no specific vent path for plate leaks tovent to the external environment, and no double seal and vent patharound the ports. Further, since the vent path is not sufficientlychanneled, leak detection can be elusive. Coupled with the state of theart construction methods, low heat transfer efficiency is a problem, aswell as low working pressure containment, due to the design. Thus,brazed-plate heat exchangers, when constructed in double-wallconfigurations, need specific features and attributes to meet safetyrequirements and a significant improvement over the state of the art.

SUMMARY OF THE INVENTION

The present invention relates to a plate heat exchanger including aplurality of nested pairs of plates, each plate of the plurality ofpairs of plates having opposed surfaces and perimeter flanges and havingsubstantially similar surface profiles. Each plate pair forms asubstantially conformal fit between contacting surfaces when pressedtogether, opposed surfaces of each plate pair providing a portion of atleast one flow path for each of at least two fluids. Facing surfaces andperimeter flanges of adjacent plate pairs of the plurality of platepairs provide a flow path boundary for two fluids of the at least twofluids. Opposed surfaces of at least one plate pair of each pair ofadjacent plate pairs provides a flow path boundary for two fluids of theat least two fluids. The at least one plate pair has a high thermalconductivity and provides a portion of the flow path boundary for twofluids of the at least two fluids, thereby providing thermalcommunication between the two fluids on the opposed surfaces of theplate. An inlet and outlet for each fluid of the at least two fluids isprovided, the inlet and outlet for each fluid being in fluidcommunication with each flow path for said fluid. A predetermined ventpath is formed in at least one of the facing surfaces of each plate paircapable of venting each fluid exterior of the perimeter flanges.

The present invention further relates to a method for making plates fora plate heat exchanger, the steps include providing a plurality ofnested pairs of plates, each plate of the plurality of pairs of plateshaving opposed surfaces and perimeter flanges and having substantiallysimilar surface profiles. Each plate pair forms a substantiallyconformal fit between contacting surfaces when pressed together, opposedsurfaces of each plate pair providing a portion of at least one flowpath for each of at least two fluids. Facing surfaces and perimeterflanges of adjacent plate pairs of the plurality of plate pairs providea flow path boundary for two fluids of the at least two fluids. Opposedsurfaces of at least one plate pair of each pair of adjacent plate pairsprovide a flow path boundary for two fluids of the at least two fluids,the at least one plate pair having a high thermal conductivity andproviding a portion of the flow path boundary for two fluids of the atleast two fluids, thereby providing thermal communication between thetwo fluids on the opposed surfaces of the plate. Each plate of pluralityof plates includes the step of forming a plurality of apertures in theplate, at least two of the apertures having an embossed regionsurrounding the apertures, each embossed region defining a path forventing fluids of the at least two fluids leaking between nested platepairs along aligned apertures of the plurality of apertures. The methodfurther includes the step of forming at least one primary vent path inthe plate, the at least one primary vent path in fluid communicationwith the at least two embossed regions for venting the at least twofluids exterior of the perimeter flanges. The method further includesthe step of selectively applying a surface treatment to at least onesurface and perimeter flanges of at least one plate, the at least onesurface corresponding to a contacting surface of a plate pair.

The present invention still further relates to a plurality of nestedpairs of plates, each plate of the plurality of pairs of plates havingopposed surfaces and perimeter flanges and having substantially similarsurface profiles. Each plate pair forms a substantially conformal fitbetween contacting surfaces when pressed together, opposed surfaces ofeach plate pair providing a portion of at least one flow path for eachof at least two fluids. Facing surfaces and perimeter flanges ofadjacent plate pairs of the plurality of plate pairs provide a flow pathboundary for two fluids of the at least two fluids. Opposed surfaces ofat least one plate pair of each pair of adjacent plate pairs provide aflow path boundary for two fluids of the at least two fluids. The atleast one plate pair has a high thermal conductivity and provides aportion of the flow path boundary for two fluids of the at least twofluids, thereby providing thermal communication between the two fluidson the opposed surfaces of the plate. An inlet and outlet for each fluidof the at least two fluids is provided, the inlet and outlet for eachfluid being in fluid communication with each flow path for said fluid. Apredetermined vent path is formed in at least one of the facing surfacesof each plate pair capable of venting each fluid exterior of theperimeter flanges.

An advantage of the present invention is a predetermined flow pathbetween each plate pair for permitting the flow of leaking fluid to theoutside environment.

A further advantage of the present invention is that the flow of leakingfluid to the outside environment occurs at a predetermined location orlocations.

A further advantage of the present invention is that a vent path isformed around each port to ensure a fluid leakage around the port sealflows to the outside environment.

An advantage of the present invention is that there is a double sealaround each port.

A still further advantage of the present invention is that the vent pathdoes not coincide with nodal connections between contacting surfaces ofadjacent plate pairs.

A yet further advantage of the present invention is that the vent pathcan be configured to reduce the level of fluid pressure required for aleaking fluid to flow to the outside environment within a predeterminedtime duration.

A further advantage of the present invention is that it can truly meetthe intent of double-wall building codes.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plate heat exchanger of the presentinvention;

FIG. 2 is a schematic exploded plan view of a plate arrangement of aplate heat exchanger of the present invention;

FIG. 3 is a plan view of one plate of a plate pair of the plate heatexchanger of the present invention;

FIG. 4 is a plan view of the other plate of the plate pair of the plateheat exchanger of the present invention;

FIG. 5 is a cross-section of a portion of a port of two adjacent, nestedplate pairs of the plate heat exchanger of the present invention;

FIG. 6 is a plan view of a plate subjected to a surface treatment of theplate heat exchanger of the present invention;

FIG. 7 is a perspective view of secondary embossments formed in theplates of the plate heat exchanger of the present invention; and

FIG. 8 is a plan view of stacked plate pairs of the plate heat exchangerof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a double-wall plate heat exchanger10 depicted in FIGS. 1-8. Such a heat exchanger is similar in principleof operation to a single-wall heat exchanger construction set forth inU.S. Pat. No. 5,462,113 issued on Oct. 31, 1995 and U.S. patentapplication Ser. No. 10/643,689 titled PLATE HEAT EXCHANGER WITHENHANCED SURFACE FEATURES, filed on Aug. 19, 2003, both incorporatedherein by reference in their entirety. The heat exchanger 10 includes aplurality of formed plates 12 comprising a high thermal conductivelymaterial such as copper disposed between a top plate 14 and a bottomplate 16 providing separated flow passages for a first fluid F1 and asecond fluid F2 while simultaneously providing thermal communicationbetween the first fluid F1 and the second fluid F2. To assist withorienting the plates 12 of heat exchanger 10, FIGS. 1 and 2 includelabeling of TOP and BOTTOM, FIG. 2 lacking the top and bottom plates 14,16. It is to be understood that the heat exchanger may be placed in avariety of physical orientations, including vertical, horizontal and anyposition therebetween. While atypical, the first and second fluids F1,F2 may have the same composition. Typically, a diametrically opposedinlet port 18 and an outlet port 20 are formed in the top plate 14permitting the first fluid F1 to access the plates 12, and similarly, adiametrically opposed inlet port 22 and an outlet port 24 are formed inthe top plate 14 permitting the second fluid F2 to also access theplates 12. Alternately, it may be advantageous to reverse theorientation of one of the pair of inlet/outlet ports so that the firstpair and second pair of fluid inlets/outlets are located on oppositeends of the heat exchanger 10.

Referring to FIGS. 1 and 2, the formed plates 12 include alternatelyarranged plate pairs 26, 28. Each plate pair 26, 28 includes similarlyarranged plates 34, 36 that are nested together, each plate 34, 36having opposed ends 30, 32. That is, the heat transfer surfaces ofplates 34, 36 are substantially similar, so that when plates 34, 36 arepressed together, the surfaces nest together to form a substantiallyconformal contact therebetween, which provides a high heat transfercoefficient through the combined thickness of the plates. Typically, theonly difference between the plate pair 26 and the plate pair 28 is thatthe ends 30, 32 are reversed, or stated alternatively, that plate pair26 is rotated 180 degrees about an axis 38 (see FIG. 1), which isperpendicular to the surface of top plate 14. Each plate 34 includes aplurality of apertures A_(U), B_(U), which align with respectiveinlet/outlet ports when the plates 34 are installed in the heatexchanger 10. Similarly, each plate 36 includes a plurality of aperturesA_(L), B_(L), which align with respective inlet/outlet ports when theplates 36 are installed in the heat exchanger 10. Depicted in anarrangement that includes inlet/outlet ports 18, 20, 22, 24, it beingunderstood that additional inlet/outlet ports can be included, such aswhen three or more heat exchange fluids are utilized. Formed in thesurface of plate 34, 36 are a plurality of V-ridges 40, also referred toas corrugations, typically arranged in a herringbone configuration toprovide a tortuous flow passage of changing direction and cross-sectionwhen arranged in adjacent plate pairs 26, 28 as discussed inabove-mentioned U.S. Pat. No. 5,462,113 and U.S. patent application Ser.No. 10/643,689 to effect thermal communication between fluids F1, F2.The plates 34, 36 extend outwardly to a flange 42 formed at the plateedges, which defines the periphery of the plates 34, 36. The flanges 42of the stacked plates 34, 36 physically touch one another and form abarrier to fluid flow to form heat exchanger 10.

The plate heat exchanger 10 of the present invention is preferably of abrazed construction, although in one embodiment, only the ports and notthe entire heat exchanger surfaces require an operation or special ringsto provide a fluid tight seal along the ports. Although not shown inFIG. 2, preferably, at least one foil plate comprised of a brazeablematerial is inserted between adjacent plate pairs 26, 28. Once the foilplates are inserted and the plates sufficiently pressed together, theheat exchanger 10 is heated to a predetermined temperature below themelting point of plates 34, 36, but above the melting point of theinserted foil plates for sufficient duration to melt the foil plates.Due to capillary action, the molten metal, preferably copper, is drawnto regions between adjacent plate pairs 26, 28 that are in contact witheach other, such as the nodes 44 (see FIG. 8), which are theintersections of the apexes of the oppositely oriented V-ridges 40 ofthe plate pairs 26, 28, and the peripheral flanges 42. The foil plates,typically comprised of copper, form metallic bonds along these regionsor nodes which are fluid tight (i.e., along the peripheral flanges), andprovide greatly increased structural support, normally expressed interms of burst pressure, which can approach 3,000 psi and sufficient towithstand pressures from the fluids F1, F2 and meet safety coderequirements.

However, during the brazing process, not only will the molten brazematerial flow between the contact surfaces of adjacent plate pairs 26,28, but other contacting surfaces as well. In other words, surfaces inconformal contact between plates 34, 36 of plate pairs 26, 28, includingthe peripheral flanges 42, can also be brazed together, unless a surfacetreatment is applied to at least one plate surface. For example,referring to FIG. 6, for plate 34, 36, the entire heat transfer surface46 is typically treated except for regions 52 surrounding the portapertures (A_(U), B_(U), A_(L), B_(L)), as well as region(s) 48 alongperipheral flange 42. By not brazing the contacting heat transfersurface 46, in case of a breach in one of the plates of plate pairs 26,28, fluid can flow through the breach and between the conformal contactsurfaces to a vent path 54 (FIG. 3) that permits the flow of the leakingfluid to the outside environment as an alert to replace the heatexchanger. The port regions 52 (FIG. 6) are masked to prevent theapplication of surface treatment material around the port apertures(A_(U), B_(U), A_(L), B_(L)) because it is desirable that a double sealis formed along the peripheries of the port apertures.

It is noteworthy that except for region(s) 48, the surface treatment isnot applied to flange 42, which permits a brazed joint to be formed onthe vast majority of the contacting flange 42 surface. The reason formasking region(s) 48 is not only to provide a leak path along the flange42 to ensure leaking fluid can escape to the outside environment, but toprovide a focused leak path by blocking all other access along theperipheral flange. Preferably, a focused inspection area, such as anembossed area 50 is formed within the non-treated region 50 to pinpointto an outside observer the expected location of any external fluidleakage. As shown in FIG. 6, a pair of opposed regions and correspondingembossed regions 50 are provided on opposite portions of the flanges 42.The embossed regions 50 are also shown in an assembled heat exchanger 10in FIG. 7. Alternately, instead of embossing the regions 50, otherindicia to indicate the location of fluid leakage that does not mar theflange surface 42, such as marking, can also be employed. Failing toprovide such a focused leak path makes leak detection more difficult,since fluid leakage could otherwise occur along any portion or portionsalong the periphery of flange 42, or spread out the possible leakageregion over an extended area of the flange.

The novel predetermined vent path of the present invention will now bediscussed. For ease of understanding, FIGS. 3 and 4 together representthe plate pair 28 with plate 34 (FIG. 3) stacked on top of plate 36(FIG. 4) when end 32 is oriented as an upper end as shown in FIG. 2.However, when end 30 is oriented as an upper end as shown in FIG. 2,FIGS. 3 and 4 represent the other plate pair 26 with plate 34 (FIG. 3)stacked on top of plate 36 (FIG. 4). As further shown in FIG. 3, a ventpath 54 is formed in plate 34 and preferably runs in a straight linefrom end 30 to end 32 to provide a predetermined path for fluid leakingbetween the conformal contact surfaces of plates 34, 36 to flow towardthe outside environment. It is to be understood that a vent path 54could be alternately or additionally be formed in plate 36 or thatmultiple vent paths 54 could be formed in each plate 34, 36. However,each vent path 54 provides a narrow region having a reduced coefficientof heat transfer through the plate pair thickness, so that a single ventpath may be preferable. It is also possible that the vent path 54 doesnot continue in a straight path from end 30 to end 32, nor is itnecessary that the vent path even provide a contiguous path from end 30to end 32 as one or more “branches” extending to one or more of theopposed ends of the plates 34, 36 that are transverse to ends 30, 32.Those skilled in the art can appreciate that increasing the number ofvent paths 54 may decrease the amount of fluid pressure required to flowthe leaking fluid and the length between the nearest vent path frompossible leak locations, but increases the amount of area having areduced heat transfer coefficient, so that an optimum arrangement can beachieved. The only limitation to the possible routing of the vent path54 is that the vent path not intersect with any of the nodes 44 (FIG.8). The profile of vent path 54 defined by a cross section that isperpendicular to the vent path can resemble an angled rooftop, curved orany other closed geometric profile so long as leaking fluid can flowalong the vent path 54 with sufficiently reduced resistance as comparedto other regions of the conformal contact surfaces between the platepair 26, 28 to flow toward and ultimately past the peripheral flanges 42to the outside environment.

It is to be understood that in addition to the profile(s) and path(s) ofvent path 54, the sizing of the vent path can also be another factor toconsider. For example, some safety regulations, such those promulgatedby UNDERWRITERS LABORATORIES INC.® or UL®, both registered trademarks ofUnderwriters Laboratories, Inc. of Chicago, Ill., or IAPMO®, aregistered trademark of the International Association of Plumbing andMechanical Officials of Ontario, Canada, specify that a leak must bevisually evident within a predetermined time duration, such as 30minutes, while operating the heat exchanger at a predetermined fluidpressure, such as 12 psi. Combinations of size, vent path routing andvent path profiles can be provided to permit these regulations to bemet. That is, these parameters can also be configured with respect toeach other so that higher fluid pressures are required so that heattransfer efficiency can be improved for example, resulting in alternateconstructions in which fluid pressures up to about 400 psi or more toabout 1 psi, including increments of 1 psi levels within this range.

It is to be understood that in the case multiple vent paths 54 areformed in the plates 34, 36, corresponding regions 48 for surfacetreatment and embossed regions 50 or other visually evident indicia canalso be added to the plate flanges 42.

In addition to providing a predetermined vent path 54, the heatexchanger 10 of the present invention includes fluid ports having adouble seal that provide enhanced protection against inadvertent mixingof fluids F1, F2. Moreover, a vent path surrounds each fluid portaperture to vent fluid leaking along any portion of any port thatbreaches the double seal in the heat exchanger to the outsideenvironment. The term port is intended to refer to the aligned openingsof the assembled plates, while the term aperture is intended to refer tothe openings in an individual plate, although the terms openings,apertures and ports may be used interchangeably. For ease ofunderstanding and for orienting the plates, FIGS. 3 and 4 togetherrepresent the plate pair 28 with plate 34 (FIG. 3) stacked on top ofplate 36 (FIG. 4) when end 32 is oriented as an upper end as shown inFIG. 2. However, when end 30 is oriented as an upper end as shown inFIG. 2, FIGS. 3 and 4 represent the plate pair 26 with plate 34 (FIG. 3)stacked on top of plate 36 (FIG. 4).

Preferably, there are two substantially identical pairs of aperturesA_(U), B_(U) formed in plate 34. Preferably, there are two substantiallyidentical pairs of apertures A_(L), B_(L) formed in plate 36. Thesubscript “U” indicates that the associated apertures (A_(U), B_(U))correspond to apertures formed in the upper plate in the plate pair 26,28. Similarly, the subscript “L” indicates that the associated apertures(A_(L), B_(L)) correspond to the lower plate in the plate pair 26, 28.Preferably, aperture A_(U) has the largest diameter of the aperturesA_(U), B_(U), A_(L), B_(L) and does not contain a peripheral embossment,while aperture B_(U) preferably has the smallest diameter of theapertures, but includes a peripheral embossment region 56. Preferably,aperture A_(L) has a diameter that is greater than the diameter ofaperture B_(U) but less than the diameter of aperture B_(L), andadditionally includes a peripheral embossment region 58. Aperture B_(L)has a diameter that is greater than the diameter of A_(L) but less thanthe diameter of A_(U) and does not include a peripheral embossment. Inother words, in a preferable construction, the diameters are sizedaccordingly: A_(U)>B_(L)>A_(L)>B_(U).

In keeping with the above convention, plate pair 28 is formed when, forexample, aperture B_(U) of plate 34 is stacked on top of and nested withaperture B_(L), which stack-up is expressed as B_(U)-B_(L). Similarly,when plate pair 28 is stacked upon nested plate pair 26, the stackingarrangement from the top plate of plate pair 28 to the bottom plate ofplate pair 26 as shown in FIG. 5, which is taken along line 5-5 fromFIG. 8 and is expressed as B_(U)-B_(L)-A_(U)-A_(L). After assembly andthe brazing operation is completed, FIG. 5 shows a partial cross sectionof a fluid port of an adjacent set of plate pairs 26, 28 as evidenced bya port centerline 66. A sealed region 60 provides a double seal toprevent a fluid flowing through a port from flowing between any of theplates of adjacent plate pairs 26, 28. The first or primary port seal isformed between plate 34 of plate pair 28 and plate 36 of plate pair 26.However, assuming sufficient braze material is etched away so that thefluid can pass between plate 34 of plate pair 28 and plate 36 of platepair 26, the secondary seal is actually two separate seals. The first ofthe two secondary seals is disposed between plate 34 and plate 36 of theplate pair 28, which prevents fluid from flowing into a peripheral ventpath 62 formed from embossed region 56 of plate 34. The second of thetwo secondary seals is disposed between plate 36 of plate pair 28 andplate 36 of plate pair 26, which prevents fluid from flowing into aperipheral vent path 64 formed from embossed region 58 of plate 36.

It is to be understood that while molten braze material flows into andforms the double seal for the ports, braze material does not flow intothe vent paths 62, 64 because the embossed regions 56, 58 are configuredso that there are insufficient capillary forces at the junctions withinregion 60 between embossed regions 56, 58 that contact opposite sides ofadjacent plate 36 of plate pair 28. It is noteworthy that the diameterA_(U) is sufficiently large so that plate 34 of plate pair 26 does notextend into the junction between embossed region 58 and plate 36 ofplate pair 28, which increases the angular spacing between the embossedregion 58 and plate 36 of plate pair 28 and provides for a moreefficient secondary port seal.

However, in the case that both primary and secondary seals surrounding aport are breached or etched away, vent paths 62, 64 ensure that theleaking fluid has a predetermined path to flow to the outsideenvironment. Referring back to FIGS. 3-5, a leaking fluid that reachesvent path 62 then flows through a vent path 68 formed in plate 34 thatbridges vent path 62 and vent path 54. Similarly, a leaking fluid thatreaches vent path 64 then flows through a vent path 70 formed in plate36 that bridges vent path 64 and vent path 54. As previously discussed,vent paths 68, 70 can extend outwardly from the respective vent path 62,64 in any direction, and include more than one path and be formed ineither or both conformal surfaces of plates 34, 36, so long as the ventpaths do not coincide with nodes 44 nodal connections.

It is to be understood that the surface treatment which prevents theformation of brazed joints can exhibit favorable thermal conductanceproperties such that the overall thermal conductance through thedouble-wall plate pairs 26, 28 is measurably enhanced. For example, asurface treatment using a special formulation of oxides can produce athermal conductance that is approximately 250 times higher than of airand approximately one half of the thermal conductance of an exampleplate material, stainless steel. Testing has revealed that this surfacetreatment material reduced the spacing between contacting heat transfersurfaces of plate pairs, which spacing being primarily due to springback of the plates, thereby enhancing the overall thermal conductancethrough the plate pairs.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A plate heat exchanger comprising: a plurality of nested pairs ofplates, each plate of the plurality of pairs of plates having opposedsurfaces and perimeter flanges and having substantially similar surfaceprofiles, each plate pair forming a substantially conformal fit betweencontacting surfaces when pressed together, opposed surfaces of eachplate pair providing a portion of at least one flow path for each of atleast two fluids, wherein facing surfaces and perimeter flanges ofadjacent plate pairs of the plurality of plate pairs provide a flow pathboundary for two fluids of the at least two fluids, and wherein opposedsurfaces of at least one plate pair of each pair of adjacent plate pairsprovide a flow path boundary for two fluids of the at least two fluids,the at least one plate pair having a high thermal conductivity andproviding a portion of the flow path boundary for two fluids of the atleast two fluids, thereby providing thermal communication between thetwo fluids on the opposed surfaces of the plate; an inlet and outlet foreach fluid of the at least two fluids, the inlet and outlet for eachfluid being in fluid communication with each flow path for said fluid;and wherein a predetermined vent path is formed in at least one of thefacing surfaces of each plate pair capable of venting each fluidexterior of the perimeter flanges.
 2. The plate heat exchanger of claim1 wherein an internal leakage of fluid between adjacent plate pairs orbetween adjacent plates of adjacent plate pairs flows along a vent pathand is visually evident exterior of the perimeter flanges when the heatexchanger is pressurized to less than about 400 psi for a predeterminedtime duration.
 3. The plate heat exchanger of claim 2 wherein the heatexchanger is pressurized to less than about 50 psi.
 4. The plate heatexchanger of claim 3 wherein the heat exchanger is pressurized to about1 psi.
 5. The plate heat exchanger of claim 1 wherein at least a portionof one of the contacting surfaces of each plate pair includes a surfacetreatment.
 6. The plate heat exchanger of claim 1 wherein the vent pathdoes not coincide with nodal points of contact between opposed surfacesof adjacent plate pairs.
 7. The plate heat exchanger of claim 6 whereinthe vent path extends in a substantially linear path toward a perimeterflange.
 8. The plate heat exchanger of claim 6 wherein the linear pathextends in a curved path toward a perimeter flange.
 9. The plate heatexchanger of claim 6 wherein the vent path includes a plurality of pathstoward a perimeter flange.
 10. The plate heat exchanger of claim 1wherein each adjacent pair of plate pairs includes a plurality of portsfor providing a flow channel for at least one fluid through the adjacentpair of plate pairs, each port of the plurality of ports having a doubleseal.
 11. The plate heat exchanger of claim 10 wherein each port has atleast two surrounding embossed regions formed in the outermost opposedplates of the adjacent plate pair in fluid communication with the ventpath.
 12. The plate heat exchanger of claim 1 wherein the plate heatexchanger is of brazed construction comprising the insertion of at leastone foil plate between the adjacent plate pairs of the plurality ofplate pairs, the at least one foil plate becoming molten and flowingbetween adjacent plates of the plurality of plates to form brazed nodalcontacts between facing surfaces of the adjacent plate pairs of theplurality of plate pairs when the plate heat exchanger is heated to apredetermined temperature below the melting point of the adjacent platepairs of the plurality of plates, but above the melting temperature ofthe at least one foil plate.
 13. The plate heat exchanger of claim 12wherein predetermined regions of plate surfaces are selectively treatedto prevent brazed contacts in the predetermined regions.
 14. The plateheat exchanger of claim 13 wherein at least one portion of at least onesurface of at least one perimeter flange of the contacting surfacesbetween the plate pairs are selectively treated to prevent the formationof brazed contacts.
 15. The plate heat exchanger of claim 14 wherein afocused inspection area substantially coincides with the at least onetreated portion.
 16. The plate heat exchanger of claim 15 wherein thefocused inspection area is an embossed region formed in the at least oneperimeter flange.
 17. A method making plates for a plate heat exchanger,the steps comprising: providing a plurality of nested pairs of plates,each plate of the plurality of pairs of plates having opposed surfacesand perimeter flanges and having substantially similar surface profiles,each plate pair forming a substantially conformal fit between contactingsurfaces when pressed together, opposed surfaces of each plate pairproviding a portion of at least one flow path for each of at least twofluids, wherein facing surfaces and perimeter flanges of adjacent platepairs of the plurality of plate pairs provide a flow path boundary fortwo fluids of the at least two fluids, and wherein opposed surfaces ofat least one plate pair of each pair of adjacent plate pairs provide aflow path boundary for two fluids of the at least two fluids, the atleast one plate pair having a high thermal conductivity and providing aportion of the flow path boundary for two fluids of the at least twofluids, thereby providing thermal communication between the two fluidson the opposed surfaces of the plate; each plate of plurality of platesincluding the step of: forming a plurality of apertures in the plate, atleast two of the apertures having an embossed region surrounding theapertures, each embossed region defining a path for venting fluids ofthe at least two fluids leaking between nested plate pairs along alignedapertures of the plurality of apertures; forming at least one primaryvent path in the plate, the at least one primary vent path in fluidcommunication with the at least two embossed regions for venting the atleast two fluids exterior of the perimeter flanges; and selectivelyapplying a surface treatment to at least one surface and perimeterflanges of at least one plate, the at least one surface corresponding toa contacting surface of a plate pair.
 18. The method of claim 17 whereinthe at least one primary vent path does not coincide with nodalconnections defined between facing surfaces of adjacent plate pairs ofthe plurality of plate pairs.
 19. A plate heat exchanger comprising: aplurality of nested pairs of plates, each plate of the plurality ofpairs of plates having opposed surfaces and perimeter flanges and havingsubstantially similar surface profiles, each plate pair forming asubstantially conformal fit between contacting surfaces when pressedtogether, opposed surfaces of each plate pair providing a portion of atleast one flow path for each of at least two fluids, wherein facingsurfaces and perimeter flanges of adjacent plate pairs of the pluralityof plate pairs provide a flow path boundary for two fluids of the atleast two fluids, and wherein opposed surfaces of at least one platepair of each pair of adjacent plate pairs provide a flow path boundaryfor two fluids of the at least two fluids, the at least one plate pairhaving a high thermal conductivity and providing a portion of the flowpath boundary for two fluids of the at least two fluids, therebyproviding thermal communication between the two fluids on the opposedsurfaces of the plate; an inlet and outlet for each fluid of the atleast two fluids, the inlet and outlet for each fluid being in fluidcommunication with each flow path for said fluid; and wherein apredetermined vent path is formed in at least one of the facing surfacesof each plate pair capable of venting each fluid exterior of theperimeter flanges.
 20. The plate heat exchanger of claim 19 wherein eachadjacent pair of plate pairs includes a plurality of ports for providinga flow channel for at least one fluid through the adjacent pair of platepairs, each port of the plurality of ports having a double seal, eachport having at least two surrounding embossed regions formed in theoutermost opposed plates of the adjacent plate pair to vent two fluidsof the at least two fluids leaking along a port between the plates ofeach adjacent plate pair to the vent path.